Makepad is neither gtk generation nor browser based. Might check the "just text/button widget" box though, I'd certainly place it on the minimalistic end of the spectrum. (haven't worked with makepad, just enjoyed the demo)
Makepad [0] is indeed quite impressive. You can directly try Makepad Studio in your browser [1]. Don't know if that's the latest or the demo version. Their demo vids are a good watch.
